Allows either rectangular or circular subdomains. This can be used to
mask or to query results from a certain sector or subdomain.

Parameters:

x1, x2, y1, y2 : float

If all four parameters are given, then a rectangular sector is
assumed that is given by two corners (x1, y1) and (x2, y2). If
only three parameters are given, then a circular sector is assumed
that is given by its center (x1, y1) and the radius x2.

Returns:

2-tuple of arrays of integers or of numpy slice objects

The indices that can be used to mask/select the grid subdomain from
an array with the same shape as the grid.